macros

问题 This question already has answers here : Qt Linker Error: “undefined reference to vtable” [duplicate] (9 answers) Closed 6 years ago . When I uncomment the Q_OBJECT macro that I need for signal-slot I get a undefined reference to vtable for MyApp error, but without the macro it compiles perfectly but I can't use signals and slots without it. I think I may be doing something stupid wrong, but please try helping because I realy can't find the problem. O and I know my code is untidy and am

问题 This question already has answers here : Qt Linker Error: “undefined reference to vtable” [duplicate] (9 answers) Closed 6 years ago . When I uncomment the Q_OBJECT macro that I need for signal-slot I get a undefined reference to vtable for MyApp error, but without the macro it compiles perfectly but I can't use signals and slots without it. I think I may be doing something stupid wrong, but please try helping because I realy can't find the problem. O and I know my code is untidy and am

问题 I'm trying to create a global list and I want it appended in a macro. Here is my setup: project \__. CMakeLists.txt \__. level1 \__. CMakeLists.txt \__. level2a \__. CMakeLists.txt \__. level2b \__. CMakeLists.txt Here is my top level CMakeLists.txt : cmake_minimum_required(VERSION 2.8) macro(listappend var) list(APPEND MY_GLOBAL_LIST "${var}") message(STATUS "LIST IN MACRO SCOPE: ${MY_GLOBAL_LIST}") endmacro(listappend) set(MY_GLOBAL_LIST "") add_subdirectory(level1) message(STATUS "LIST: $

问题 How can I define a macro (or a workaround for this) where the parameter is at the beginning of the line? #define SINGLETON_IMPLEMENTATION(className) \ ##className* ##className::instance_ = NULL; This give a compiler warning (GCC 3.2.3): " '##' cannot appear at either end of a macro expansion" 回答1: ## is the concatenation operator; the compiler is just complaining about that. You cannot concatenate a token without something before it, i.e. at the beginning of the macro expansion; just try to

问题 Here's the code: #include <stdio.h> #include <stdio.h> #define VAL1(a,b) a*b #define VAL2(a,b) a/b #define VAL3(a,b) ++a%b int main() { int a = 1; int b = 2; int c = 3; int d = 3; int e = 5; int result = VAL2(a,d)/VAL1(e,b)+VAL3(c,d); // result = 1 //int result = a/d/e*b+++c%d; // result = 0 printf("%d\n", result); return 0; } Why aren't the results of two statements the same? 回答1: In one case you have + ++ and in the other case you have ++ + . + ++ and ++ + are different streams of tokens .

问题 Here's the code: #include <stdio.h> #include <stdio.h> #define VAL1(a,b) a*b #define VAL2(a,b) a/b #define VAL3(a,b) ++a%b int main() { int a = 1; int b = 2; int c = 3; int d = 3; int e = 5; int result = VAL2(a,d)/VAL1(e,b)+VAL3(c,d); // result = 1 //int result = a/d/e*b+++c%d; // result = 0 printf("%d\n", result); return 0; } Why aren't the results of two statements the same? 回答1: In one case you have + ++ and in the other case you have ++ + . + ++ and ++ + are different streams of tokens .

问题 I have these macros which generate error in Visual Studio 2015. #define log_params __FILE__, __LINE__ #define log(file, line, message, ...) _snprintf_s(nullptr, 0, 0, message, __VA_ARGS__) Now calling this never works log(log_params, "testing %d", 4) Any thoughts? I also checked output of preprocessor and it is: _snprintf_s(nullptr, 0, 0, 4 ); EDIT 1 Intresting finding #define log(file, line, message, ...) file line will produce this : "service.cpp", 164 "testing %d" Is it normal? 回答1: The

问题 I want to use a global constexpr variable: constexpr int foo = 123; instead of a C macro: #define FOO (123) in some code I'm writing. I would like to be guaranteed the same behavior, in the sense that this will not take up space in memory at runtime, nor will it be visible/exist in the compiled object code (that is, its value will be used as an immediate where relevant). Can I get this guarantee at all? Under some conditions? Assume, of course, I'm not trying to use x's address or any such

问题 Are the two below constructions equivalent in assumption that "cond" is not conflicting with any name in the program #define DOWHILE do { (some_code); } while (0) #define FORLOOP for(bool cond = true; cond; cond = false) (some_code) The purpose of this question is: I have something like this bool printLogs; // nonconstant dynamic variable And I have a macro(I cannot do big changes, it is a big project; I have to deal with this macro) #define LOG ... which is used like LOG << "message" << 1 <<

问题 I am writing code for an Arduino. The code starts to get long so I want to use some debugging macros, and to be able to show some debugging information on serial port. For example, I have some macros to define which serial ports I use: #define SERIAL_GPS Serial1 #define SERIAL_IRIDIUM Serial2 #define SERIAL_VN100 Serial3 How can I write a macro to show the ports I use for each one? I.e. some macro that would print on a debug serial port: Port for GPS: Serial1 Port for Iridium: Serial2 Port